Bolometer 4.27 Direct sensing bolometer. Samuel Langley in 1880 used a bolometer made of two iron strips each 7 mm long, 0.177 mm wide, and 0.004 mm thick. Unlike later bolometers, which use a separate absorber and a temperature sensor, the resistance of the strips was measured directly, with one strip exposed to radiation and the second kept at ambient temperature. The dif- ference in resistance between the strips was used as an indication of tem- perature. The bolometer described above is used to measure IR radiation from a star. At an ambient temperature of 25 C, the smallest resistance difference between the strips measurable is 0.001 2. Calculate the lowest power density of radiation measurable. Assume absorption efficiency of 85% and a time constant of 0.8 s. Properties of iron are: conductivity: 1.0 x 10 S/m, TCR city: 0.46 J/g/K. 0.0065/C, density: 7.86 g/cm', specific heat capa- %3D